Circuit arrangement and method for operating a fingerprint sensor

ABSTRACT

The circuit arrangement or the operating method ensure that the fingerprint sensor is started automatically when a sufficiently high-contrast image can be produced. To this end, test runs are carried out in which only pan of the pixels, for instance along a column or line of a matrix-like array in a square raster, are taken into account. It is determined whether the difference between the maximum and minimum values of the sensor signals exceeds a predetermined value. In said case, the normal scanning procedure of the fingerprint sensor is started.

[0001] In the case of a fingerprint sensor, a raster-shaped arrangementof individual sensor elements that are assigned to the points of theimage to be detected is usually present in one plane, and electroniccircuit components for determining a signal are connected thereto. Theindividual sensor elements that are set up for capacitive measurement,for example, are driven in series, and this can be done by row or columnin the case of a rectangular raster and thus of a matrix-likearrangement of the pixels. To date, a fingerprint sensor is alwaysstarted anew and delivers a complete fingerprint image after each start.This is done independently of how rich in contrast the image producedturns out to be. In particular, fingerprint images are also suppliedwhenever the finger is not yet correctly resting on the support surfaceof the sensor. It would be desirable, on the other hand, for it to bepossible to operate the sensor in a fashion ensuring that completefingerprint images are delivered only whenever a finger rests on thesupport surface such that a clear and contrasty image can be produced.

[0002] It is the object of the present invention to specify apossibility as to how a fingerprint sensor can be used always to producesufficiently contrasty images when a finger is rested on.

[0003] This object is achieved with the aid of a circuit arrangementhaving the features of claim 1 and with the aid of the method having thefeatures of claim 2. Refinements follow from the dependent claims.

[0004] The circuit arrangement according to the invention or theoperating method according to the invention ensure for a fingerprintsensor that the fingerprint sensor is started automatically when afinger is rested on in such a way that a sufficiently contrasting imagecan be produced. This is achieved by virtue of the fact that test runsare carried out in the case of which only a portion of the pixels istaken into account, and the sensor signals determined are used toestablish whether it is possible to achieve a sufficient contrast. Thecircuit arrangement according to the invention generates for thispurpose the difference between a maximum and a minimum value of thesensor signals, for example along a column or a row of a matrix-likearrangement of pixels. If this difference is sufficiently large, whichindicates a sufficient contrast of an image to be produced, the normalscanning operation of the fingerprint sensor with the aid of whichcomplete fingerprint images are produced is started. This ensures thatthe complete fingerprint images produced are of satisfactory quality.

[0005] The method according to the invention therefore provides fordetermining contrasts within a previously selected group of pixels, andto initiate the production of a complete fingerprint image in the eventof sufficient contrast.

[0006] A more precise description of an exemplary embodiment of thecircuit arrangement and of the method is given below with the aid of theattached drawings.

[0007]FIG. 1 shows in plan view a scheme of a matrix-like arrangement ofsensor elements.

[0008]FIG. 2 shows a scheme for a circuit arrangement according to theinvention.

[0009] A raster-shaped arrangement in the scheme of a matrix isillustrated in FIG. 1 for the sensor elements of a fingerprint sensor.Sensor surfaces 2 for the individual pixels are located inside a regionprovided as support surface 1 for a finger. A column of pixels of thismatrix is highlighted with a dashed line and two arrows 3. In apreferred exemplary embodiment of the circuit arrangement according tothe invention or the method according to the invention, the sensorsignals of the sensor elements of such a column are determined in eachcase along this column in successive runs. The maximum and minimumvalues occurring in a run supply a measure of the achievable contrasts.If the difference between the maximum and minimum values overshoots afixed value prescribed in accordance with the design of the fingerprintsensor and/or of the method, the driving of the sensor elements ismodified such that it is no longer only signals of individual columnsthat are generated, but an entire fingerprint image is produced.

[0010] The individual sensor signals of the pixels respectivelyevaluated in this form are generated in the same way as in the case ofthe taking of the entire fingerprint image. The sequence of the drive ismodified in accordance with the invention by comparison with thesequence in the case of the taking of an entire image such that in eachcase only the pixels of a row, column, diagonal or similar selectedgroup of pixels are detected, and their sensor signals are evaluated.Disregarding this additionally prescribable driving sequence, the drivecircuit provided for this purpose can otherwise be designed in the wayknown per se for fingerprint sensors.

[0011]FIG. 2 shows a preferred refinement of a circuit arrangementaccording to the invention, with the aid of which the method accordingto the invention can be executed. A sensor signal determined in eachcase in relation to the respective pixel passes to the input 4 of thecircuit. Located there is a circuit node 5 with the aid of which thesignal is supplied to two comparing elements 6, 9. It is established ineach case in these comparing elements whether the current sensor signalis greater or smaller than the stored maximum or minimum value. Thesestored values are located in a first and second memory register 12, 15.The value stored there is fed via lines 8, 11 to the comparing elementsand compared there with the current sensor signal. If it is establishedthat the current sensor signal is, for example, greater than the storedmaximum value, the first comparing element 6, which is illustrated onthe left-hand side in the circuit diagram of FIG. 2, is used via anassociated first control line 7, which leads to a first gate circuit 13,to accept the current signal, which is fed via the node 5 to the gatecircuits 13, 16 that are present, into the first memory register 12. Thesecond comparing element 9 correspondingly has a second control line 10and a second gate circuit 16. The respective current minimum value isstored here.

[0012] Subsequent lines 14, 17 are used to supply the stored values toone subtracting element 20, 21 each, which can, as indicated in thedrawing, for example, be an adding element upstream of which there isconnected on the side of the second memory register 15 provided for theminimum value, a reversing element 21 for the sign. The difference thatis produced in the subtracting element is supplied to a third comparingelement 22 in which this difference is compared with a specificprescribed signal level 23. Output at the output 24 is a control signalwith the aid of which the complete production of the fingerprint imageis initiated, and the difference between the maximum and the minimumvalues overshoots the constantly prescribed value. Present forrespective subsequent runs are circuit parts 18, 19 with the aid ofwhich the contents of the memory registers are reset or erased. Theminimum and maximum values are thus always detected only respectivelyfor one run, that is to say for in each case exactly one interrogationof the sensor signals along a column or row.

[0013] The evaluated pixels need not [lacuna] along a row but, forexample, lie in that region of the sensor surface in which the methodaccording to the invention supplies the best results. This can be asmall section in the middle of a sensor surface. The method can beadapted to the respective circumstances and requirements by selectingthe pixels to be detected according to the invention.

1. A circuit arrangement for operating a fingerprint sensor, in the caseof which a circuit is present for detecting sensor signals of individualpixels and for controlling a sequence in which the sensor signals aredetected, characterized in that present as further circuit parts are afirst comparing element (6) that determines whether a supplied sensorsignal is greater than a stored first value, a second comparing element(9), that determines whether a supplied sensor signal is smaller than astored second value, a first memory register (12), in which the firstvalue is stored, a second memory register (15), in which the secondvalue is stored, a first gate circuit (13) that, in the event of theoccurrence of a larger value than the value stored in the first memoryregister, causes this larger value to be stored, a second gate circuit(16), that, in the event of the occurrence of a smaller value than thevalue stored in the second memory register, causes this smaller value tobe stored, a subtracting element (20, 21), in which the differencebetween the values stored in the memory registers (12, 15) is formed,and a third comparing element (22), in which this difference is comparedwith a prescribed value, and the production of a fingerprint image isinitiated in the case when the difference is greater than the prescribedvalue.
 2. A method for operating a fingerprint sensor, in the case ofwhich a) a sensor signal is generated for each of the pixels of apreviously selected group of pixels, b) a maximum and a minimum value ofthese sensor signals are determined, c) the steps a and b are carriedout in repeated runs, account being taken in step b only of values of arun just performed, and d) the production of a fingerprint image isinitiated upon the occurrence of a difference between maximum andminimum values that overshoot a prescribed value.
 3. The method asclaimed in claim 2, in which in step a the sensor signals relating tothe pixels are determined in each case in a permanently prescribedsequence, and in step b a respective current maximum value and arespective current minimum value are stored, each newly determinedsensor signal is compared with the stored values, and in the event ofundershooting or overshooting of one of the stored values the currentsensor signal is stored in a fashion replacing the relevant value. 4.The method as claimed in claim 3, in which there are determined for amatrix-like arrangement of pixels the sensor signals relating to pixelsof a row, column or diagonal in a sequence corresponding to thearrangement of the pixels in the relevant row, column or diagonal.